Sodium carbonate is an effective detergency builder which can be used wholly or partially to replace sodium tripolyphosphate (STP) in detergent powders, but it has disadvantages with respect to the production of spray-dried powders having satisfactory physical properties. STP is an outstandingly good matrix or "building block" material for carrying the organic components, for example, surfactants, of a detergent composition, and also gives powders of good structure, that is to say, powders consisting of strong, non-friable agglomerates of the primary particles formed during spray-drying. Sodium carbonate, unlike STP, is a poor matrix material: under normal ambient conditions it is constantly picking up and losing moisture as conversion from anhydrous salt to monohydrate and vice versa takes place.
It has now been discovered that the incorporation of succinic acid, or certain other acids, in free acid form in a slurry containing sodium carbonate causes its transformation into sodium sesquicarbonate of a crystal size and morphology that render it especially effective as a powder matrix. On spray-drying, a powder containing needle-like crystals of sodium sesquicarbonate having excellent matrix or "building block" properties is obtained. While succinic acid is not the only acid that may be used, it is an especially beneficial choice since the other product of its reaction with sodium carbonate in the slurry is sodium succinate which is itself an excellent structurant. Another preferred acid is linear alkylbenzene sulphonic acid, in which case the other product of the reaction is the detergent active material, sodium linear alkylbenzene sulphonate.
The use of succinic acid salts as structurants in powders built with aluminosilicates has already been proposed. EP 61 295B (Unilever) discloses detergent powders built with zeolite and structured with water-soluble salts of succinic acid. Low or zero phosphate powders low in silicate and structured with water-soluble salts of succinic acid and anionic polymers are disclosed in our copending application claiming the priority of British Patent Application No. 85 26999 filed on Nov. 1, 1985.
The present invention is relevant to the production of whole detergent powders, purely inorganic carrier materials intended for incorporation in detergent powders, or any intermediate product.